Method for controlling an internal combustion engine

ABSTRACT

A method for controlling an internal combustion engine ( 1 ) comprising a number (W) of cylinders ( 3 ); the control method comprises determining the total target torque (C i     —     obj ) required to be delivered for the operation of the internal combustion engine ( 1 ); determining a number (Wa) of active cylinders ( 3 ) to be controlled in use for injection and combustion; while a number (Ws) of cylinders ( 3 ) are not active and are not controlled for injection and combustion, but only for aspirating an air mass; determining the required target torque (C i     —     obj ) to be delivered for the operation of the internal combustion engine ( 1 ) for each of the active cylinders ( 3 ); and controlling the internal combustion engine ( 1 ) as a function of the required target torque (C i     —     obj ) to be delivered for the operation of the internal combustion engine ( 1 ) for each of the active cylinders ( 3 ).

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims priority to Italian PatentApplication BO2012A 000322 filed on Jun. 12, 2012.

BACKGROUND OF INVENTION

1. Field of Invention

The present invention relates to a method for controlling an internalcombustion engine, in particular a supercharged engine.

2. Description of Related Art

As known, some internal combustion engines are provided with aturbocharger supercharging system, which can increase the powerdeveloped by the engine by exploiting exhaust gas enthalpy forcompressing the air aspirated by the engine, and thus increasingvolumetric intake efficiency.

A turbocharger supercharging system comprises a turbocharger providedwith a turbine, which is arranged along an exhaust pipe to turn at ahigh speed under the bias of the exhaust gases expelled by the engine,and with a compressor, which is turned by the turbine and is arrangedalong the air feeding pipe to compress the air aspirated by the engine.

A rather evident turbo lag usually occurs when a considerable, sudden,rapid increase of torque or power is requested in low torque or powerconditions (low rpm and slow speed, i.e. when the driver floors theaccelerator pedal, e.g. for overtaking). Turbo lag is the tendency ofengines with turbocharger of failing to respond with power to the fastpressing of the accelerator pedal, and is particularly annoying in thecase of sports car applications, in which the turbocharger superchargingsystem allows to achieve high performance.

Turbo lag is mainly caused by the inertia moment of the rotor, andoccurs in case of sudden, rapid request for more torque or power andbecause the pressure must increase in the overall volume of the circuitdownstream of the compressor.

Various solutions have been suggested over the years to attempt toreduce turbo lag and to further improve the performance of enginesprovided with turbocharger. For instance, a variable geometryturbocharger or a turbocharger comprising a plurality of turbines inserial or parallel configuration etc. may be used. All the solutionsknown until now are in all cases particularly disadvantageous in termsof costs and overall dimensions.

It is the object of the present invention to provide a method forcontrolling an internal combustion engine, in particular supercharged bymeans of a turbocharger, which control method is easy and cost-effectiveto implement.

SUMMARY OF INVENTION

The invention overcomes the disadvantages in the related art in a methodfor controlling an internal combustion engine, in particularsupercharged by means of a turbocharger provided with a turbine and witha compressor is provided.

Objects, features, and advantages of the invention are readilyappreciated as the invention becomes better understood while asubsequent detailed description of embodiments of the invention is readtaken in conjunction with the accompanying drawing thereof.

BRIEF DESCRIPTION OF EACH FIGURE OF DRAWING OF INVENTION

FIG. 1 diagrammatically illustrates an embodiment of an internalcombustion engine supercharged by means of a turbocharger and providedwith an electronic control unit which implements a control methodaccording to the present invention;

FIG. 2 shows the comparison of the performance of the internalcombustion engine supercharged by means of a turbocharger in a firstnormal configuration and in a second configuration achieved according tothe present invention on a characteristic engine rpm/ETASP plane atvarious ratios;

FIG. 3 is a block chart which diagrammatically shows the operation ofthe control method according to the present invention in a superchargedinternal combustion engine provided with intake valves controlledaccording to a variable lift law; and

FIG. 4 is a block chart which diagrammatically shows the operation ofthe control method according to the present invention in a superchargedinternal combustion engine provided with intake valves controlledaccording to a fixed lift law.

DETAILED DESCRIPTION OF EMBODIMENTS OF INVENTION

In FIG. 1, numeral 1 indicates as a whole an internal combustion enginesupercharged by a turbocharger supercharging system 2.

The internal combustion engine 1 comprises four cylinders 3, each ofwhich is connected to an intake manifold 4 by means of at least onerespective intake valve (not shown) and to an exhaust manifold 5 bymeans of at least one respective exhaust valve (not shown). The intakemanifold 4 receives fresh air (i.e. air coming from the externalenvironment) through an intake pipe 6, which is provided with an aircleaner 7 and is adjusted by a throttle 8. An intercooler 9 for coolingthe intake air is arranged along the suction pipe 6. An exhaust pipe 10,which feeds the exhaust gases produced by combustion to an exhaustsystem, is connected to the exhaust manifold 5, which exhaust pipe emitsthe gases produced by the combustion into the atmosphere and normallycomprises at least one catalyzer 11 and at least one silencer (notshown) arranged downstream of the catalyzer 11.

The supercharging system 2 of the internal combustion engine 1 comprisesa turbocharger 12 provided with a turbine 13, which is arranged alongthe exhaust pipe 10 to turn at high speed under the bias of the exhaustgases expelled from the cylinders 3, and a compressor 14, which isarranged along the intake pipe 6 and is mechanically connected to theturbine 13 in order to be rotatably fed by the turbine 13 itself andincrease the pressure of the air fed into the intake pipe 6.

A bypass pipe 15 is arranged along the exhaust pipe 10 and is connectedin parallel to the turbine 13 so that its ends are connected upstreamand downstream of the turbine 13 itself. A wastegate valve 16 isarranged along the bypass pipe 15, is adapted to adjust the exhaust gasflow flowing through the bypass pipe 15 and is driven by an actuator 17.A bypass pipe 18 is arranged along the exhaust pipe 6 and is connectedin parallel to the compressor 14 so that its ends are connected upstreamand downstream of the compressor 14 itself. A Poff valve 19 is arrangedalong the bypass pipe 18, adapted to adjust the exhaust gas flow flowingthrough the bypass pipe 18 and driven by an actuator 20.

Explicit reference will be made to an internal combustion engine 1supercharged by means of a turbocharger 12 in the description thatfollows. Alternatively, the control method described above may beadvantageously applied to any internal combustion engine, superchargedby means of a dynamic or volumetric compressor, for instance.

According to a first variant, the internal combustion engine 1 isprovided with an electric machine mechanically connected to theturbocharger 12 and set up to recover exhaust gas energy; in thisvariant, it is possible both to deliver the necessary torque byconsuming electricity and to brake by delivering electricity.

Alternatively, the control method described above can be applied to asupercharged internal combustion engine of the type described in patentapplication EP-A1-2096277, which comprises a turbine, a compressormechanically independent from the turbine, an electric generatorrotatably fed by the turbine for generally electricity and an electricmotor which rotatably feeds the compressor.

In general, the embodiments described above have in common the fact thatthere is an air volume between the compressor 14 and the cylinders 3.The mass of air entrapped in each cylinder 3 for each engine cycle maybe regulated by means of a respective intake valve (not shown) by meansof a valve actuation device with a variable opening law, such as anelectromagnetic or electrohydraulic camless actuation device.Alternatively, the mass of air entrapped in each cylinder 3 for eachengine cycle is regulated by means of the interposition of a valve 27,in an embodiment, a throttle, between the compressor 14 and the intakevalves. As a possible further alternative, a valve actuation device withvariable opening law may be provided and a valve 27, in an embodiment, athrottle, may be inserted between the compressor 14 and the intakevalves.

The internal combustion engine 1 is controlled by an electronic controlunit 21, which governs the operation of all the components of theinternal combustion engine 1, including the supercharging system 2. Inparticular, the electronic control unit 21 drives the actuators 17 and20 of the wastegate valve 16 and of the Poff valve 19. The electroniccontrol unit 21 is connected to sensors 22, which measure thetemperature T_(o) and the pressure P_(o) along the intake pipe 6upstream of the compressor 14, to sensors 23, which measure thetemperature and pressure along the intake pipe 6 upstream of thethrottle 8, and to sensors 24, which measure the temperature andpressure inside the intake manifold 4. Furthermore, the electroniccontrol unit 21 is connected to a sensor 25, which measures the angularposition (and thus the rotation speed) of a crankshaft of the internalcombustion engine 1, and to a sensor 26, which measures the timing ofthe intake and/or exhaust valves. It is similarly worth noting that nosensors adapted to measure the rotation speed of the turbocharger 12 areneeded.

The strategy implemented by the electronic control unit 21 to controlthe supercharged internal combustion engine 1 will be described below.In particular, the electronic control unit 21 is set to increase themass and volumetric flow of air/and of exhaust gas which cross thecompressor 14 and the turbine 13, with respect to the air flow actuallyused by the supercharged internal combustion engine 1 in combustion togenerate the desired power. In order to implement the aforesaid controlstrategy, the electronic control unit 21 is set up to differentiatecylinder management 3, in particular to differentiate the intake airflow and the air trapped inside each cylinder 3, and to differentiatethe operating mode.

In other words, the strategy includes generating the target torquerequired by the vehicle driver using only some of the firing cylinders3, while the remaining cylinders 3 aspirate as much air as possible. Forexample, in a supercharged internal combustion 1 with four cylinders 3,two cylinders 3 are active and deliver the required torque by aspiratingan air mass which is approximately double the one of the air mass whichthey would aspirate in normal operating conditions (i.e. if all fourcylinders 3 were active). The remaining two cylinders 3 are not activeand are controlled to aspirate the maximum air but are not involved incombustion.

The performance of the internal combustion engine 1 in two differentconfigurations was empirically determined on a test bench for asupercharged internal combustion engine 1 provided with four cylinders 3(in third gear), in which in a first configuration there are four activefiring cylinders 3 (hereinafter named “normal configuration”) and inwhich in a second configuration there are two active firing cylinders 3and two cylinders 3 which aspirate but which are not involved ininjection or combustion (hereinafter named “scavenging configuration”).

As shown in FIG. 2, at various ratios, an increase of supercharging upto 210 mbar can be observed in scavenging configuration on thecharacteristic rpm/ETASP plane, wherein engine rpm is from 900 to 1800rpm and ETASP is the intake efficiency, defined by the ratio between theair trapped in each cylinder 3 for each cycle and the air mass whichfills the displacement in the normal configuration. A 210 mbar increaseof supercharging is equal to approximately two/three times thesupercharging which can be obtained in normal configuration. Inscavenging configuration, ETASP intake efficiency, defined by thetrapped air mass for each cycle and for each cylinder 3 and the air masswhich fills the displacement in normal configuration, is an averagevalue on the four cylinders 3, i.e. is approximately half the one in theactive cylinders 3.

Assuming the condition of a supercharging internal combustion engine 1at constant rpm (e.g. 3000 rpm) the supercharged internal combustionengine 1 in normal configuration with the four active firing cylinders 3aspirates a mass air flow Mc which allows to deliver a power Pc, with asupercharge pressure p_c and a torque Cc transmitted to the drivewheels. The operating point c in normal configuration is identified onthe plane which represents the characteristic curves of the turbocharger14 (usually supplied by the manufacturer of the turbocharger 12) for agiven revolution speed Nc, to which a predetermined position of thewastegate valve WGc 16 corresponds.

It has been found that the supercharged internal combustion engine 1 inscavenging configuration with two active firing cylinders 3 aspirates amass air flow Ma and allows to achieve a supercharge pressure p_a and,at the same time, deliver the same power Pc and the same torque Cctransmitted to the drive wheels as normal configuration, to which apredetermined position of the wastegate valve WGa 16 corresponds.

In particular, the mass air flow Ma of the scavenging configuration ishigher than the mass air flow Mc of the normal configuration, and thewastegate valve 16 is more closed than in normal configuration (in otherwords, WGa<WGc).

In the variant in which the mass air flow Ma of the scavengingconfiguration is approximately double the mass flow rate Mc of thenormal configuration, it has been found that a supercharge pressure p_ain scavenging configuration which is approximately double thesupercharge pressure p_c of normal configuration can be obtained. It isworth noting that this increase of supercharge pressure may be reachedby delivering the same power Pc and transmitting the same torque Cc tothe drive wheels as the normal configuration.

In use, in the moment in which, when requested by the driver, the torqueCc delivered to the drive wheels must be increased, the electroniccontrol unit 21 is set up to control the switch from scavengingoperating mode to normal operating mode. In other words, all fourcylinders 3 become active, fire, can immediately deliver the alreadyavailable power corresponding to the air flow rate Ma and transmit therequired torque to the drive wheels. In this manner, it is possible toreduce in considerable manner the response time of the turbocharger 12(the so-called turbo lag) because the air circuit of the superchargedinternal combustion engine 1 is already pressurized and the turbocharger12 is already at full rate (in other words, the inertia andpressurization time of the entire supercharging circuit are nearlycancelled out).

As known, the active zone of the operating range of the compressor 14 islimited, on the left part of the reduced mass flow rate/compressionratio plane, by a pumping line which delimits the forbidden zone andconsists of the sets of points in which the aerodynamic balance in thecompressor 14 is interrupted and there is a periodical, noisy andviolent rejection of flow rate to the mouth, the effects of which may bedestructive for the blades of the compressor 14 itself.

Also near the pumping line and assuming the condition of a supercharginginternal combustion engine 1 at constant rpm (e.g. 3000 rpm) in normalconfiguration, it is possible to determine that the four active firingcylinders 3 aspirate a mass air flow Mc which allows to deliver a powerPc, with a supercharge pressure p_c and a torque Cc transmitted to thedrive wheels. The operating point c in normal configuration isidentified on the pumping line on the plane which represents thecharacteristic curves of the turbocharger 14 (usually supplied by themanufacturer of the turbocharger 12) for a given rpm Nc, to which apredetermined position of the wastegate valve WGc 16 corresponds.

It has been found that the supercharged internal combustion engine 1 inscavenging configuration with only two cylinders 3 active and firingaspirates a mass air flow Ma and allows to reach a supercharge pressurep_a and, at the same time, allows to deliver the same power Pc and thesame torque Cc transmitted to the drive wheels as in normalconfiguration. In particular, it has been found that the mass flow rateMa in scavenging configuration is higher than the mass flow rate Mc innormal configuration.

It is immediately apparent that also in this case, a superchargepressure p_a in scavenging configuration higher than the superchargepressure p_c in normal configuration can obtained without the occurrenceof pumping. It is worth noting that this increase of superchargepressure may be achieved by delivering the same power Pc andtransmitting the same torque Cc to the drive wheels as normalconfiguration.

A supercharged internal combustion engine 1 of the type described abovecomprises a number W of cylinders 3, wherein Wa are the active cylinders3 which by firing generate the required torque, while Ws indicates theremaining cylinders 3 which do not generate the required torque byfiring and are actuated only to aspirate a mass air flow.

According to a variant, both the number W of cylinders 3 and the numberWa of active cylinders 3 are even and appropriately timed to limit theoscillations which are transmitted to the drive shaft.

The chain which from the torque request to the device which acts on theaccelerator pedal can provide the combustion air mass of each cylinder 3is shown in FIG. 3 and may be diagrammatically illustrated as follows:

a) the user acts on the accelerator pedal and the torque Ceobj requiredfrom the drive shaft can be determined by means of maps stored in theelectronic control unit 21 and of rpm;

b) the torque Ceobj required from the drive shaft is added to thefriction, pumping and auxiliary torques so as to obtain the total actualtorque Ciobjt required from the drive shaft;

c) the actual torque Ciobj′ for each active cylinder 3 is thuscalculated by the ratio between the actual total torque Ciobjt requiredfrom the drive shaft and the number Wa of active firing cylinders 3;

d) the actual torque Ciobj′ for each active cylinder 3 is divided by theEGR efficiency etaegr (if present), by the mixture efficiency etalambdaand by the spark advance efficiency etasa, thus providing the referenceactual torque Ciobj for each active cylinder 3; and

e) from the reference actual torque Ciobj for each active cylinder it ispossible to determine the combustion air mass mobj for each activecylinder 3.

In particular, in step e), the firing air mass mobj for each activecylinder 3 is calculated by the consumption of air of the engineCsrif(Ciobj,n) in reference conditions (i.e. with a stoichiometricmixture A/F stech and optimal advance), for each cylinder 3, as follows:

$\begin{matrix}\begin{matrix}{{Cs} = {{mc}/{Lu}}} \\{= {{mc}/\left( {{Cmi}*4\; \pi} \right)}} \\\left. {= {{m/\left( {A/F} \right)}{stech}*{Cmi}*4\; \pi}} \right)\end{matrix} & \lbrack 1\rbrack\end{matrix}$

wherein:

Cs: specific consumption;

Lu: mechanical work for each cycle and for each cylinder 3;

mc: fuel mass for each cycle and for each cylinder 3;

m: air mass for each cycle and for each cylinder 3; and

Cmi: indicative positive torque for each cylinder 3.

The following can be obtained from equation [1]:

m=Cs(Cmi,n)*(A/F)stech*Cmi*4π

i.e. it is possible to directly map the air consumption of the cylinder3 as a function of the actual torque Cmi for the cylinder 3 and of theengine speed n:

m=f3(Cmi,n)=Csrif(Cmi,n)

Where f3 or Csrif are the functions by means of which determining thefuel air mass mobj for each active cylinder 3 which must be burnttogether with the fuel mass mc in stoichiometric ratio conditions andoptimal advance to obtain the reference actual torque Ciobj required foreach active cylinder 3 (with EGR zero, i.e. with unitary EGRefficiency).

Similarly, it is possible to determine the actual drive torque Ciobj foreach active cylinder 3 as a function of a combustion air mass mobj foreach active cylinder 3 as follows:

Ciobj=g(mobj,n)

Where g is the function by means of which determining the referenceactual torque Ciobj for each active cylinder 3 in stoichiometric ratioconditions, optimal advance and with unitary EGR efficiency, burning thefiring air mass mobj for each active cylinder 3 together with thecombustion mass mc.

In general conditions in step b), the torque Ceobj required from thedrive shaft is added to the friction, pumping and auxiliary torques soas to obtain the total actual torque Ciobjt required from the driveshaft as follows:

$\begin{matrix}{{Ciobjt} = {{Ceobj} + {friction}}} \\{= {g\mspace{14mu} \left( {{mobj},n} \right)*{etalambda}\mspace{14mu} ({lambda})*}} \\{{{etasa}\mspace{14mu} \left( {{sa\_ ottimo}\text{-}{sa}} \right)*{etaegr}*{wa}}}\end{matrix}$

Wherein, friction is the sum of the mechanical, pumping losses andauxiliary friction torques, and wherein etasa is the combustionefficiency relative to the actuated spark advance (i.e. as a function ofthe deviation with respect to the optimal advance which is stored inspecific maps in the electronic control unit 21), while etalambda is theefficiency as a function of the mixture, and etaegr is the efficiency asa function of the EGR mass.

According to a variant, it is possible to determine the reference actualtorque Ciobj for each active cylinder 3 as a function of a combustionair mass mobj for each active cylinder 3 as follows:

Ciobj=k(n)*mobj+offset(n).

According to a first variant, the supercharged internal combustionengine 1 comprises intake valves which are controlled by the electroniccontrol unit 21 independently for each cylinder 3 with variable lift.For example, each valve is provided with an electrohydraulic actuator,or alternatively with an electromagnetic actuator for controllingclosing and/or lift.

A map is stored in the electronic control unit 21 as a function of theengine point which is identified by the rpm and by the load. For eachengine point, the map provides the number Wa of active cylinders 3, theactuating methods of the solenoid valve (typically to be chosen betweenlate LO opening, early EC closing etc.) and the relative pressure in thecommon rail 5.

The cylinders 3 which are not active and which do not fire may insteadbe used to aspirate an air mass which, as previously described, mayallow to improve performance in terms of supercharging of the internalcombustion engine 1. Also in this case, a map is stored in theelectronic control unit 21 as a function of the engine point which isidentified by the rpm and the load. For each engine point, the mapprovides the target scavenge mass which represents the quantity of airwhich optimizes the performance of the supercharged internal combustionengine 1.

The chain which from the torque request from the users acting on theaccelerator pedal can provide the target scavenge mass Mscav of eachcylinder 3 is shown in FIG. 3 and may be diagrammatically illustrated asfollows:

the user acts on the accelerator pedal and the total actual torqueCiobjt required from the drive shaft (obtained from the torque Ceobjrequired from the drive shaft added to the friction, pumping andauxiliary torques) can be determined by means of the maps stored in theelectronic control unit 21;

the total actual torque Ciobjt required from the drive shaft allows todetermine, by means of the engine rpm and the previously defined map,the target pressure value Pobj upstream of the intake valve, which(saturated by the atmospheric pressure Pa) provides the targetsupercharge pressure Ptobj_eng. The target supercharge pressure valuePtobj_eng is added to such a supercharge reserve RDS which allows tooptimize the performance of the internal combustion engine 1. Explicitreference is made to the description of patent application BO2011A000400for a better understanding of the control strategy of a superchargedinternal combustion engine by means of a supercharge reserve. Theminimum target supercharge pressure value Ptobj_eng, which is added to asupercharge reserve RDS, thus allows to determine the target superchargepressure value Ptobj;

from the total actual torque Ciobjt requested from the drive shaft thenumber of active cylinders Wa, the actuating method of the solenoidvalves (typically to be chosen from late LO opening, early EC closingetc.), the actuating method of the inactive cylinders and the targetscavenge mass Mscav can be determined (using maps and rpm) as describedabove;

finally, the target scavenge mass mscav for each of the inactivecylinders 3 can be obtained from the target scavenge mass Mscav.

The actual torque Ciobj′ for each active cylinder 3 is determined by theratio between the total actual torque Ciobjt required from the driveshaft and the number Wa of active cylinders determined by the aforesaidmaps also as a function of the engine ratio rpm. Such actual torqueCiobj′ for each active cylinder 3 is divided by EGR efficiency (ifpresent), the mixture etalambda efficiency and the spark advance etasaefficiency and provides the reference actual torque Ciobj, which bymeans of the specific consumption map Csrif (or f3) determines thetarget air mass mobj for each active cylinder 3. The target scavengemass mscav for each of the inactive cylinders 3 may be simply obtainedfrom the ratio between the target scavenge mass Mscav and the valuewhich corresponds to the number of inactive cylinders 3 (i.e. the totalnumber of cylinders, minus the number Wa of active cylinders) multipliedby the number of revolutions (rpm), multiplied by 0.5.

Finally, two filling models (of the speed density type, for instance)are stored in the electronic control unit 21 by means of which the angleto be actuated for the solenoid valve which control the number of activecylinders Wa and the remaining inactive cylinders, respectively, can bedetermined.

In particular, a first filling module is made to determine the angleVanga for controlling the solenoid valves arranged at the activecylinders. The filling module includes a plurality of input data,including: the combustion air mass mobj for each active cylinder 3, therpm, the actuating method of the solenoid valves (typically, to bechosen between late LO opening, early EC closing etc.), the pressurevalue P upstream of the intake valve in the common rail 5, and othersurrounding conditions (such as, for example, the air temperature in thecommon rail 5 and the temperature of the coolant used in thesupercharged internal combustion engine 1).

A second filling model is made to determine the angle Vangs forcontrolling the solenoid valves arranged at the active cylinders. Thefilling module includes a plurality of input data, including: the targetscavenge mass mscav for each inactive cylinder 3, the rpm, the actuatingmethod of the solenoid valves of the inactive cylinders 3, the pressurevalue P upstream of the intake valve in the common rail 5, and othersurrounding conditions (such as, for example, the air temperature in thecommon rail 5 and the temperature of the coolant used in thesupercharged internal combustion engine 1).

According to a variant, in order to determine the frictions and thepumping work, the number Wa of active cylinders 3 and the number ofinactive cylinders 3 is taken into account.

It is apparent that the control strategy described above must be adaptedto the operating conditions in transient ratios (or to operatingconditions other than standard or stationary operating conditions).

In particular, during the transient step, the method includesdetermining at each predetermined step of calculating the actualpotential torque which is calculated as follows:

Ce_pot_temp=g(mamax,n)*etasa*etalambda*etaegr*wtemp−friction(wtemp,w)

Wherein:

Ce_pot_temp: actual potential torque which corresponds to the torquewhich can be achieved with the maximum air that can be trapped in thecylinder at the current pressure and temperature;

n: rpm;

mamax: maximum quantity of air which can be trapped in the cylinder 3;

g: is the function by means of which the reference actual torque Ciobjis determined for each active cylinder 3 in stoichiometric ratio,optimal advance and with unitary EGR efficiency, burning the combustionair mass mamax for each active cylinder 3 together with the fuel massmc;

friction: sum of the mechanical frictions and the pumping losses;

wtemp: the current number of firing cylinders 3, i.e. the current numberof active cylinders 3.

In an initial condition:

Wtemp=Warif

wherein:

Wtemp: temporary number of firing cylinders 3, i.e. temporary number ofactive cylinders; and

Warif: reference number of cylinders 3 determined as a function of theengine point.

The electronic control unit 21 then checks the condition:

Ceobj≦Ce_pot_temp

Wherein:

Ce_pot_temp: potential actual torque; and

Ceobj: the torque required from the drive shaft.

If the aforesaid condition occurs, then:

Wa=Wtemp=Warif

wherein Wtemp and Warif have the meaning introduced above with regardsto the equation and Wa is the number of active controlled, and thuscurrent, cylinders 3.

If the aforesaid condition does not occur (i.e. the torque Ceobjrequired from the drive shaft is higher than the actual potential torqueCe_pot_temp), then the current number of firing cylinders 3 isincreased, i.e. the current number of active cylinders 3. In otherterms:

Wtemp=Wtemp+Δ

wherein Δ is the number of additional active cylinders 3 which fire.

Also in this case, according to an embodiment, the number Δ ofadditional cylinders 3 is even, in an embodiment, equal to 2, andappropriately timed to limit the oscillations which are transmitted tothe drive shaft.

Once that the current number of firing cylinders 3, i.e. the currentnumber of active cylinders 3, has been increased, the Ceobj≦Ce_pot_tempcondition is checked again and the process is stopped only when such acondition is true, or when the current number Wa of firing cylinders 3,i.e. the number of current active controlled cylinders Wa, is equal tothe number W of cylinders 3.

By means of the methods described above, it is possible to converge tothe situation in which the number Wa of active cylinders 3 allows tooptimize the performance of the supercharged internal combustion engine1, i.e. to converge to Warif. If in transient ratio or in non-standardconditions, the number Warif of active reference cylinders 3 should notbe sufficient to generate the torque required by the driver, theelectronic control unit 21 is in all cases ready to control the firingof some or all of the remaining cylinders 3 for an interval of time toallow to generate the required torque.

According to a possible variant, the equation introduced above fordetermining the actual potential request torque Ce_pot_temp is replacedby the equation wherein the efficiency etaegr is replaced by the maximumefficiency etaegrmax which allows to optimize the delivered torque. Inparticular:

Ce_pot_temp=g(mamax,n)*etasa*etalambda*etaegrmax*wtemp-friction(wtemp,w)

Wherein:

Ce_pot_temp: actual potential required torque, which corresponds to themaximum torque which can be required;

n: rpm;

mamax: maximum quantity of air which can be trapped in the cylinder 3and which is calculated by means of the filling model above with thecurrent pressure value and the current temperature value; and

g: function by means of which determining the indicative positive torqueCiobj (reference) for each active cylinder 3 in stoichiometric ratioconditions, optimal advance and zero EGR, burning the combustion airmass mamax for each active cylinder 3 together with the fuel mass mc;

friction: sum of the mechanical frictions and the pumping losses;

Wtemp: the current number of firing cylinders 3, i.e. the current numberof active cylinders.

According to this variant, it is possible to minimize the number Wa ofactive cylinders using the EGR efficiency formula which maximizes thedelivered torque; this is possible if the EGR actuator is sufficientlyfast.

According to a further embodiment, it is possible to use an equation, inwhich the efficiency etaegr is replaced with the maximum efficiencyetaegrmax, which allows to optimize the delivery torque, and theefficiency etasa is replaced with the optimal advance etasamax, whichallows to optimize efficiency with the object of reducing the number Waof active cylinders 3.

In particular:

Ce_pot_temp=g(mamax,n)*etasamax*etalambda*etaegrmax*wtemp−friction(wtemp,w).

According to a further embodiment, an equation can be used, in which theefficiency etaegr is replaced with the maximum efficiency etaegrmax,which allows to optimize the delivered torque, the advance etasa isreplaced with the optimal advance etasamax, and the mixture etalambda isreplaced with the mixture etalambdamax, which allows to optimize thedelivered torque with the target of reducing the number Wa of activecylinders 3. In particular:

Ce_pot_temp=g(mamax,n)*etasamax*etalambdamax*etaegrmax*wtemp−friction(wtemp,w).

Furthermore, it is worth noting that during a transient step, the targetscavenge mass Mscav, which represents the quantity of air whichoptimizes the performance of the internal combustion engine 1, may beappropriately increased (or decreased) with respect to a targetreference scavenge mass Mscavrif defined as a function of the enginepoint of the electronic control unit 21.

For example, the target scavenge mass Mscav is calculated as follows:

Mscav=Mscavrif(C,n)+Δ(derivative(Ciobjt)).

Wherein:

Mscavrif(C,n): reference target scavenge mass, as a function of theengine operation point determined by the load and by the number ofrevolutions per minute; and

Ciobjt: total actual torque required from the drive shaft.

According to a second variant, the supercharged internal combustionengine 1 comprises throttle intake valves, which are controlled by theelectronic control unit 21 with fixed lift law and/or by a VVT (VariableValve Timing) device, which hydraulically acts on the shaft whichactuates the throttle intake valves by modifying the inclination thereofwith respect to the drive shaft.

A map is stored as a function of the engine point which is identified bythe rpm and the load (which, according to a total drive torque variantCiobjt required from the drive shaft or by the intake efficiency ETASP)in the electronic control unit 21. For each engine point, the mapprovides the timing angle of the valves during intake stroke, the timingangle of the valves during the exhaust stroke and the external EGR mass.

Furthermore, a map is stored as a function of the engine point which isidentified by the rpm and the load (which, according to a total drivetorque variant Ciobjt required from the drive shaft or by the intakeefficiency) in the electronic control unit 21. For each engine point,the map provides the number Warif of active reference cylinders 3.During a preliminary step, the target number Waobj of active cylinders 3is equal to the number Warif of reference active cylinders 3.Furthermore, it is worth noting that it is not possible to control thetarget scavenge mass Mscav for each of the inactive cylinders 3.

A filling model is stored the electronic control unit 21 (e.g. of thespeed density type) by means of which it is possible to determine theair mass trapped in each cylinder 3 at each cycle. In particular, thefilling module includes a plurality of input data, including: rpm,timing angle of the valves during intake stroke, timing angle of thevalves during exhaust stroke, the value of the pressure P upstream ofthe intake valve in the common rail 5 and other surrounding conditions(such as, for example, the temperature of the air in the common rail 5and the temperature of the coolant used in the supercharged internalcombustion engine 1).

According to a variant, in order to determine the friction and thepumping work, the number W of cylinders 3 and the target number Waobj ofactive cylinders 3 is taken into account.

It is apparent that the control strategy described above must be adaptedto the transient ratio operating conditions (or to operating conditionsother than standard or stationary operating conditions).

In particular, the method includes determining the actual torque at eachpredetermined calculation step as follows:

Ce=g(ma,n)*etasa*etalambda*etaegr*wtemp−friction(wtemp,w)

Wherein:

Ce: actual torque;

n: rpm;

ma: air mass trapped for each cylinder 3 at each cycle which iscalculated by means of the filling model shown in the equation;

g: function by means of which to determine the indicative positivetorque Ciobj (reference) for each active cylinder 3 in stoichiometricratio conditions, optimal advance and zero EGR, burning the combustionair mass ma for each active cylinder 3 together with the fuel mass mc;

friction: sum of the mechanical frictions and the pumping losses; and

wtemp: temporary number of firing cylinders 3, i.e. the current numberof active cylinders.

In an initial condition the current number Wtemp of firing cylinders 3is determined, i.e. the current number of active cylinders, equal to thereference number Warif of cylinders 3 determined as a function of theengine point.

The electronic control unit 21 thus proceeds by determining the advanceefficiency etasa which would be needed to obtain the actual targettorque with the temporary number Wtemp of firing cylinders 3. In orderto determine the advance efficiency etasa which would be needed toobtain the actual target torque, the mass ma of air trapped in eachcylinder 3 is maintained and calculated by means of the equationreference module, the EGR efficiency etaegr and the current mixtureetalambda. During a preliminary step of setting up and tuning, theminimum and maximum values are established for the advance efficiencyetasa, which are indicated using etasamin and etasama, respectively. Ifthe advance efficiency etasa is comprised between the minimum andmaximum values, indicated by etasamin and etasamax of the advanceefficiency, respectively, then the number Wa of active cylinders 3 isequal to the temporary number Wtemp of firing cylinders 3, i.e. thetemporary active number of cylinders, i.e. again to the number Warif ofreference cylinders 3 determined as a function of the engine point.

If the aforesaid condition occurs, then Wa is set equal to Wtemp and toWarif, wherein Wtemp and Warif have the meaning illustrated above forthe equation and Wa is the number of active controlled cylinders 3.

There are two situations if the aforesaid condition does not occur (i.e.if the advance efficiency etasa is not comprised between the minimum andthe maximum values, indicated by etasamin and etasamax of the advanceefficiency, respectively).

In the first case, the advance efficiency etasa is lower than theminimum advance efficiency value etasamin. In this case, the currentnumber of firing cylinders 3 is decreased, i.e. the current number ofactive cylinders 3. I.e. Wtemp=Wtemp−Δ, wherein Δ is the number ofactive cylinders 3 which are subtracted from firing. At this point, theelectronic control unit 21 thus proceeds by determining again theadvance efficiency etasa which would be needed to obtain the actualtarget torque with the temporary number Wtemp of firing cylinders 3. Theprocess stops when the advance efficiency etasa is comprised between theminimum and maximum advance values etasamin and etasamax. The number Waof active cylinders 3 is equal to the temporary number Wtemp of activecylinders 3, i.e. the number Warif of reference cylinders 3 determinedas a function of the engine point decreased by a number of activecylinders 3 which are subtracted from firing.

In the second case, the advance efficiency etasa is higher than themaximum advance efficiency value etasamax. In this case, the temporarynumber of firing cylinders 3 comes, i.e. the temporary number of activecylinders 3. I.e. Wtemp=Wtemp−Δ, wherein Δ is the number of activecylinders 3 which are added to firing. At this point, the electroniccontrol unit 21 thus proceeds by determining again the advanceefficiency etasa which would be needed to obtain the actual targettorque with the temporary number Wtemp of firing cylinders 3. Theprocess stops when the advance efficiency etasa is comprised between theminimum and maximum advance values etasamin and etasamax. The number Waof active cylinders 3 is equal to the temporary number Wtemp of activecylinders 3, i.e. the number Warif of reference cylinders 3 determinedas a function of the engine point increased by a number of activecylinders 3 which are added to combustion.

In the first case, the process is interrupted when the temporary numberWtemp of firing cylinders 3, i.e. the temporary number of activecylinders, is equal to a number Wmin of active cylinders 3 which isdefined during an initial step of setting and tuning and is variableaccording to the engine point.

In the second case, the process obviously stops when the temporarynumber Wtemp of firing cylinders 3, i.e. the temporary number Wtemp ofactive cylinders is equal to the number W of cylinders 3.

Also in this case, according to a variant, the number Δ of activecylinders 3 which are respectively either added to or subtracted fromcombustion is even, in an embodiment, equal to 2, and appropriatelytimed to limit the oscillations which are transmitted to the driveshaft.

By means of the method described above, it is possible to converge tothe situation with the number Wa of active cylinders 3 which allows tooptimize the performance of the supercharged internal combustion engine1 and also to determine the necessary spark advance (by means of aninverse function of the advance efficiency etasa which is stored in theelectronic control unit 21).

According to a possible alternative embodiment, the case may occur(particularly in stationary conditions) in which the spark advance SA isdifferent, in particular smaller, than the reference spark advance, i.e.the optimal spark advance which allows to optimize consumptions. Inother words, it may occur that SA<SAottimo, i.e. that the advanceefficiency etasa is lower than the maximum advance efficiency valueetasamax. In this case, the target number Waobj of cylinders 3 is setequal to the number Warif of reference active cylinders 3 determined asa function of the engine point increased by a number of active cylinders3 which are added to firing only for a limited, predetermined intervalof time. In this manner, it is possible to gradually increase the numberWaobj of active cylinders 3 to reach the condition in which the sparkadvance SA is approximately equal to the optimal spark advance SAottimoand wherein the target number Waobj of active cylinders 3 is equal tothe number Wa of active cylinders 3.

According to a further variant, it is possible to inject a quantity offuel into the number Ws of inactive cylinders (3).

According to a further variant, it is possible to control the number Wsof inactive cylinders 3 for combustion but not for torque generation.

The two variants described above essentially allows to increase theexhaust temperature in order to approximately heat up the catalystand/or to provide more power to the turbine 13 of the turbocharger 12(in particular in the case of spark-ignited engines).

The case of a supercharged internal combustion engine 1 comprising anynumber of cylinders 3 was taken into account in the description above.In particular, the description above is advantageously, but notexclusively applied to an internal combustion engine 1, supercharged bymeans of a turbocharger 12 and provided with four cylinders 3 with acrankshaft offset by 180°, and wherein the firing sequence is 1-3-4-2 insequence. In this case, active cylinders 3 are 1-4 or 3-2, or 1-3-4-2,respectively.

Furthermore, according to a possible variant (not shown), the internalcombustion engine 1 is supercharged by means of a number ofturbochargers 12. For example, in the case of an internal combustionengine 1 supercharged by means of two turbochargers 12, the electroniccontrol unit 21 is set up to control as active cylinders 3 those whichare connected to a first turbocharger 12; while the electronic controlunit 21 is set up to control as inactive cylinders 3 not involved incombustion those which are connected to a second turbocharger 12.

The description above is advantageously, but not exclusively applied toa supercharged internal combustion engine 1. Indeed, the control methoddescribed above may be advantageously applied to an aspirated internalcombustion engine 1.

A particular case occurs when the scavenge mass Mscav is zero, i.e.Mscav=0, or in the case of aspirated engine; in the latter case, ifPobj>Patm, then the number of firing cylinders 3 is increased in thechain by a determined value Δ to reach the condition in which Pobj≦Patm.

Furthermore, the strategy described above may be advantageously appliedif implemented for controlling supercharged internal combustion engineby means of a supercharge reserve and is provided with a sporty styledriving detector of the type described in Italian patent BO2012A000216.

The method described above has many advantages. In particular, despitebeing advantageous in terms of costs, easy and cost-effective toimplement and not implying an excessive additional computing burden forthe electronic control unit 21, it allows to reduce turbo lag and tooptimize the performance of the engine with turbocharger 12.

The invention has been described above in an illustrative manner. It isto be understood that the terminology that has been used above isintended to be in the nature of words of description rather than oflimitation. Many modifications and variations of the invention arepossible in light of the above teachings. Therefore, within the scope ofthe appended claims, the invention may be practiced other than asspecifically described above.

What is claimed is:
 1. A method for controlling an internal combustionengine (1) supercharged by a turbocharger (12) provided with a turbine(13) and with a compressor (14) and further comprising a number (W) ofcylinders (3) which are connected to a same intake manifold by arespective intake valve and to a same exhaust manifold (5) by arespective exhaust valve, the control method comprising steps of:determining the total target torque (C_(i) _(—) _(objt)) required to bedelivered for the operation of the internal combustion engine (1);determining a number (Wa) of active cylinders (3) to be controlled, inuse, for injection and combustion, wherein the number (Ws) of activecylinders (3) is any number comprised between 0 and the number (W) ofcylinders (3); determining a number (Ws) of inactive cylinders (3) whichare, in use, controlled to aspirate the most air possible so as toincrease the supercharging pressure; determining the target torque(C_(i) _(—) _(obj)) required to be delivered for the operation of theinternal combustion engine (1) for said number of active cylinders (3)to be controlled, in use, for injection and combustion; and controllingthe internal combustion engine (1) as a function of the target torque(C_(i) _(—) _(obj)) required to be delivered for the operation of theinternal combustion engine (1) for said number of active cylinders (3)to be controlled, in use, for injection and combustion.
 2. A methodaccording to claim 1, wherein at least one of the number (Wa) of activecylinders (3) to be controlled for injection and combustion and thenumber (W) of cylinders (3) are even.
 3. A method according to claim 1,wherein the step of determining the total target torque (C_(i) _(—)_(objt)) required to be delivered for the operation of the internalcombustion engine (1) comprises the further steps of: determining anactual torque (Ceobj) required to be delivered for the operation of theinternal combustion engine (1) by means of the requests of the useracting on the accelerator pedal; determining a plurality of correctionparameters, such as friction, pumping work and auxiliary loads; anddetermining the total target torque (C_(i) _(—) _(objt)) required to bedelivered for the operation of the internal combustion engine (1) as afunction of the actual torque (Ceobj) required to be delivered for theoperation of the internal combustion engine (1) and of the correctionparameters.
 4. A method according to claim 1, wherein the step ofdetermining the target torque (C_(i) _(—) _(obj)) required to bedelivered for the operation of the internal combustion engine (1) foreach one of the active cylinders (3) comprises calculating the ratiobetween the total actual torque (Ceobj) requested to the drive shaft andthe number (Wa) of active firing cylinders (3).
 5. A method according toclaim 1 and comprising the further steps of: determining at least one ofthe applied spark advance efficiency and the EGR efficiency and themixture efficiency; and determining the combustion air mass (mobj) foreach active cylinder (3) as a function of at least one of the appliedspark advance efficiency and of the EGR efficiency and of the mixtureefficiency and as a function of the target torque (C_(i) _(—obj) )required to be delivered for the operation of the internal combustionengine (1) for each one of the active cylinders (3).
 6. A methodaccording to claim 1 and comprising the further step of injecting fuelinto said number (Ws) of inactive cylinders (3).
 7. A method accordingto claim 1 and comprising the further step of controlling said number(Ws) of inactive cylinders (3) for combustion but not for torquegeneration.